I admit I had to look up tree collards—new to me! And I got to reading about them. Hopefully someone here who has actually grown them will respond to you. Meanwhile, I don't know if you've seen this website, but these folks are serious about tree collards and provide a TON of detail plus a video about starting them from cuttings. Apparently it's a s l o w process. Hope it helps.
https://www.projecttreecollard...
